Small 21 KB PDF covering the 4N39 optocoupler, catalogued as a schematic. The name states only the part number, so the content is most likely a brief datasheet with pinout, LED input ratings and output characteristics. The 4N39 is an optoisolator with a thyristor-type output used to switch loads from isolated control circuitry. Use the sheet to verify pin order and trigger current when testing or replacing the device, and compare it with the larger combined 4N39/4N40 document if details are missing.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @cyberlanelectonik. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.